Let's try this one more time shall we?
The Hebrew bible, Jewish scripture, is commonly referred to as the Tanakh. Basically, the Tanakh is what you refer to as the OT. Tanakh is a Hebrew acronym for the three sections that comprise it, which in English are the Torah, the Prophets and the Writings. Got that so far?
In order of sacredness the Torah is the most sacred, the Writings are the least. From the traditional vantage, as someone once put it, the Torah is words by G-d, the Prophets are words from G-d, and the Writings are words about G-d.
If you ask why does the Torah not include Joshua through Malachi - which by the way is the Christian arrangement of Hebrew scripture, not the arrangement within Judaism, the answer depends, in part, on how you view how and when those writings became part of the canon. For example, traditional Judaism believes in Torah m'Sinai, that the Torah was dictated by G-d to Moses.